The use of capacitive sensors to measure a change in dimension of an object such as a telescoping device having a variable dimension are known. Moser in U.S. Pat. No. 4587850 discloses apparatus for detecting and measuring the motion of a piston in a cylinder, including a variable impedance comprising a dielectric moving between the fixed electrodes. The extent of the motion of the dielectric into the space between the electrodes provides a measure of the motion of the piston. The change in the magnitude of the capacitor is determined using a capacitance bridge, such as a Wheatstone bridge. A reference capacitor is also provided to compensate for changes in the variable capacitor arising from changes in temperature and other ambient conditions. Measuring circuitry such as that disclosed by Moser can be realized using a microprocessor to switch in various combinations of capacitors for the variable capacitor while balancing the bridge. A look-up table in the microprocessor then provides an output code for the voltage corresponding to the value of the measured capacitance. This type of circuit is difficult to reduce to a single monolithic integrated circuit due to the large values and number of capacitors used and requires extra components to produce a linear electrical output with change in dimension or distance traveled.
It would be desirable to have an electronic system to measure the variation in capacitance having an electrical output which is linear with the change in capacitance and which can be implemented in an integrated circuit.